Composition comprising silver metal particles and a metal salt

ABSTRACT

The invention pertains to a composition comprising silver metal particles and an additive, characterized in that the additive is a metal salt or a mixture of metal salts wherein the metal is yttrium or a metal of group 2 of the Periodic System, and wherein the composition comprises &lt;2 atom % of the metal. Preferably, the salt is a salt of magnesium acetate. The compositions can be used to make temperature-resistant electrically-conductive silver-containing layers, for instance for use in AMLCD.

The invention pertains to a composition comprising silver metal particles and an additive, to temperature-resistant electrically-conductive layers, to an active matrix liquid crystalline display (AMLCD) comprising the said layers, and to the use of said composition in the manufacture of silver-containing layers in an article.

Compositions comprising silver particles with or without other constituents, are well known in the art. For instance, in EP 826,415 a method for preparing a silver sol has been disclosed. These silver sols are used for making a conductive film on a substrate. By spin-coating the sol onto a substrate, such films are made and these are then heated at 150° C.

In EP 276,459 a method for manufacturing cathode-ray tubes was disclosed. Antistatic silicon dioxide films were prepared that contained small amounts of (among others) silver. To a solution or a colloidal solution containing metal particles a cationic or anionic surfactant was added as an additive for improving the stability of the solution, after which an antistatic film was produced by a spraying, dispensing, or dipping method of this material onto a substrate, followed by heating at 200° C. for 15 min.

It was found that silver-containing layers, particularly layers that contain 80 vol. % or more silver, cannot be heated above 250° C., and preferably not above 200° C. without serious appearance of irreversible hillocking/exfoliating and/or creep phenomena. Creep is a process wherein the film deteriorates into a plurality of small sections containing silver and section therein between containing no silver. On creep the surface roughens, which means that the mirror-like appearance of the silver-containing layer disappears. Silver-containing layers (or films) that have undergone exfoliation and/or creep no longer have a low resistivity, and because of the low conductivity have become unsuitable for most of the applications that require a resistivity of less than 6 μΩ.cm (microOhm.centimeter). Since silver-containing layers are usually made of compositions also comprising organic materials such as organic binder materials or stabilizers, and because only relatively low temperatures (less than 250° C., preferably less than 200° C.) can be used for making silver-containing layers with sufficient conductivity, such layers usually contain amounts of organic materials that are not sufficiently removed at those temperatures. Also in further processing steps high temperatures (higher than 250° C.) may be necessary. For instance, fritting a CRT-cone to a screen occurs at 450° C. To make it possible to use silver at such high temperatures, a polymeric binder and frit glass particles can be mixed into a silver paste. However, the presence of the glass particles reduces the conductivity considerable. Furthermore, since these added particles have sizes in the micron range, such mixed pastes are not longer suitable to make silver-containing layers thinner than 1 μm. Thin layers of well-conducting silver on an insulating surface, such as on Corning® 1737 glass, which are used for active matrix liquid crystal displays (AMLCD), or on glass or any other substrate for use in infrared reflective stacks, can be made by using an electroless silver process, but the maximum temperature in the further reaction steps is limited to 250° C. At that temperature Si₃N₄ deposition takes place, and moreover, the electroless process is commercially not desired anyway since it is a slow non-equilibrium process with a short bath pot life.

There is a considerable need for a method of making thin conductive silver-containing layers that can be heated at temperatures higher than 250° C. without losing its conductivity and mirror-like appearance.

In an earlier non pre-published international application having application number IB02/04461 (PHNL010800) it was found that compositions comprising silver metal particles and an additive, which is a silane derivative comprising at least one methyl group and at least one alkoxy group, are very stable and can be applied onto a substrate and heated to temperatures as high as 450° C., usually as high as 700° C., and in many instances even as high as 1000° C. It was further found that the resistivity of silver-containing layers of 200 nm thickness made of these compositions at these high temperatures remained as low as 2.5 to 6 μΩ.cm, which values are completely comparable with the resistivity of silver-containing layers that are obtained by electroless depositing silver at 250° C., which was found to be 3-4 μΩ.cm. According to the examples of this patent application the spin-coated glass plates were cured at 250° C. for 30 min, followed by a further heat treatment for 30 min at 350, 450, 500, and 550° C. The results obtained with these silver-containing layers with respect of resistivity and layer thickness were depicted in the figures of said patent application. As can be seen, it was found that the above-mentioned method only leads to a resistivity lower than 4 μΩ.cm when the heat treatment was performed at about at least 400° C. It now appeared that this temperature poses a problem when, for instance, a common LCD display has to be made. Glass that is normally used in such displays as a substrate for the active layer has a heating upper limit of about 350° C. At higher temperature deformation of the substrate may occur. In practice this means that silver layers with sufficient conductivity cannot be prepared on common glass substrates. Therefore there is a need to improve the former silver metal particles and silane derivative additive method by searching for new additives that possess similar anti-creep properties, but that can be applied onto heat-sensitive substrates that can be heated at temperatures between 250° C. and 400° C., obtaining a layer without losing its conductivity and mirror-like appearance.

It was found that use of a composition comprising silver metal particles and an additive, characterized in that the additive is a metal salt or a mixture of metal salts, wherein the metal is yttrium or a metal of group 2 of the Periodic System, and wherein the composition comprises <2 atom % of the metal, solves the above-mentioned problem. The commonly usable metals salts therefore are selected from salts of Y, Mg, Ca, Sr, and Ba. Most preferred metal salt is a magnesium salt.

The silver-containing layers of the present invention still show improved adherence to inorganic substrates such as glass. It was further found that this composition was of particular use when applying a heat treatment between 250 and 450° C., more preferably between 250 and 350° C. These profitable properties make silver-containing films of the composition of the invention very suitable for application in AMLCD, passive integration, high temperature-resistant reflective layers (such as used in Fast Intelligent Tracking—FIT), and in reflective displays using substrates that cannot stand temperatures higher than about 350° C. It is stressed that addition of magnesium salt to silver has been described by Lee et al., Journal of the Korean Physical Society, 40(1), January 2002, 110-114. However, these authors have added as much as 5 atom % of Mg to the silver using a sputtering process. If such amounts of magnesium are introduced in silver layers according to the present invention, the obtained layers will contain crystallized magnesium salt and are unsuitable to be applied as silver layers in displays and the like. It was now found that it is essential to make layer containing less than 2 atom % of the metal in silver to obtain layers that can be used in displays.

The metal salt can be an inorganic salt such as metal oxide, metal nitrate, metal carbonate, metal phosphate, and the like or an organic salt such as metal acetate, metal propionate, metal citrate, metal stearate, metal tartrate, and the like. It is also possible to use mixtures of such salts, including mixtures of inorganic and organic salts. Preferably, these metal salts are magnesium salts, more preferably an organic salt of magnesium or magnesium nitrate. Most preferably the magnesium salt is magnesium acetate or magnesium nitrate. Preferably, the composition comprises <1 atom % of the metal, most preferably 0.25 to 0.5 atom %. At concentrations higher than 2 atom % the conductivity decreases as the result of disturbance of the silver metallic structure.

The most common composition of the invention is a colloidal silver sol comprising the metal salt These sols are very stable and have a long pot life. Application of these sols onto a substrate can simply be performed in the manners known in the art, for instance by a spin-coating or printing process.

The composition is usually brought onto a substrate, usually an inorganic substrate such as glass to form a temperature-resistant electrically-conductive silver-containing layer. These layer can be used, inter alia, in an active matrix liquid crystalline display (AMLCD).

In general, the compositions are also very suitable for making silver-containing layers for use in a device wherein the silver-containing layer is exposed to a temperature of 250° C. to 350° C. Such conductive layers can also be applied to an article, such as an AMLCD, a organic electroluminescent (display) device or plasma display panel, or in IR reflective stacks.

The invention is illustrated by the following examples.

In the drawings:

FIG. 1 shows a graph of the resistivity R (in μΩ cm) versus the curing temperature T (in ° C.) of a silver layer in accordance with the invention (triangles) and not in accordance with the invention (squares); and

FIG. 2 shows a graph of the relative density d (in dimensionless units) versus the curing temperature T (in ° C.) of a silver layer in accordance with the invention (triangles) and not in accordance with the invention (squares).

EXAMPLE

Magnesium acetate was added to a dispersion of silver colloids in water (obtained from Nippon Paint; sol contains 17 wt. % of silver and 13 wt. % of organic stabilizer) to obtain 0.25 atom % of magnesium in the silver. The mixture was diluted with water to obtain the desired viscosity.

COMPARATIVE EXAMPLE

MTMS-containing silver layers were made by mixing MTMS (methyl trimethoxy silane) and silver colloids in water (obtained from Nippon Paint as above) according to the method as described in NL 010800 to obtain a suspension containing 3 atom % of silicon in the silver.

The layers of the Example and the comparative example were deposition onto a Corning® 1737 substrate by spin-coating and dried at 80° C. for 5 min. After a pre-cure at 210° C. in air the samples were cured in a tube oven under a 100% oxygen atmosphere. The heating rate was 25° C./min and the samples were held at the end temperature for 30 min. After cooling the sheet resistance was measured using a 4-point contact probe and the layer thickness was measured using an Alpha Step, from which values the resistances were calculated. RBS measurements were used to obtain quantitative measurements of silver. In FIG. 1 the resistivity is depicted as function of the curing temperature. In FIG. 2 the densification of the silver layer is depicted as function of the curing temperature.

The metal salts Mg(NO₃)₂, Ca(NO₃)₂ Sr(NO₃)₂ Ba(NO₃)₂ and Y(Ac)₃ provided results similar the exemplary Mg-salt Co(Ac)₂ also provided good results.

Resistance to corrosion may be enhanced by adding further metal salts such as a copper and/or a palladium salt For example, an “APC” coating of silver comprising 0.9 at % Pd and 1.7 at % Cu can thus obtained by wet deposition instead of sputtering. 

1. A composition comprising silver metal particles and an additive, characterized in that the additive is a metal salt or a mixture of metal salts wherein the metal is yttrium, cobalt or a metal of group 2 of the Periodic System, and wherein the composition comprises <2 atom % of the metal.
 2. The composition of claim 1 wherein the metal is magnesium.
 3. The composition of claim 2 wherein the metal salt is magnesium nitrate or a salt of magnesium and an organic base.
 4. The composition of claim 3 wherein the metal salt is magnesium acetate.
 5. The composition of claim 1 wherein the composition comprises <1 atom % of the metal.
 6. The composition of claim 1 wherein the silver metal particles are a colloidal silver sol.
 7. An electrically-conductive layer comprising a silver composition-containing layer, the composition comprising silver and a metal salt or a mixture of metal salts wherein the metal is a metal of group 2 of the Periodic System, and wherein the composition comprises <2 atom % of the metal.
 8. An article comprising the electrically-conductive layer of claim
 7. 9. An active matrix liquid crystalline display comprising the electrically-conductive layer of claim
 7. 10. Use of the composition of claim 1 in the manufacture of silver-containing layers. 